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Molecular cluster calculations for the analysis of N2 chemisorption on nickel surfaces
L26
Surface Science 179 (1987) L26-L32 North-Holland, Amsterdam
SURFACE SCIENCE LETTERS MOLECULAR CLUSTER CALCULATIONS FOR THE ANALYSIS OF N 2 CHEMISORIYrION ON NICKEL SURFACES W U Yue a n d C A O Pei-lin Department of Physics, Zhejiang University, Hangzhou, People's' Rep. of China Received 6 June 1986; accepted for publication 8 September 1986
Self-consistent Hartree-Fock-Slater molecular cluster calculations for the chemisorption of nitrogen on nickel surfaces have been performed. The clusters are NisN 2, NivN2 and NigN 2 for N 2 on Ni(100), (111) and (110) surfaces respectively, with the N-N distance equal to the free molecular value. The theoretical results for the binding energy show that the atop chemisorption on Ni(ll0) is stronger than on the other two surfaces. The optimized Ni-N bond length is 1.80 on Ni(ll0) and 1.85 A on the others. In the ground-state valence levels, 46 is the main bonding orbital. In the total DOS and difference curve for NigN 2, the 48 and 1~ + 56 peaks are located at 11.9 and 8.1 eV below E v, in good agreement with the UPS results. The N 2 2ff induced small peaks are around 2.5 eV below EF, which is related to the tendency of N 2 dissociation. The charge transfer in N 2 chemisorption is done by a donation-backdonation scheme.
I n our previous works we have analyzed the c h e m i s o r p t i o n of C O a n d N O o n t r a n s i t i o n metal surfaces b y X a - D V M (discrete variational m e t h o d ) [1-3]. T h e c h e m i s o r p t i o n of C O o n t r a n s i t i o n metal surfaces has been e m p l o y e d as the p r o t o t y p e system of molecular chemisorption. We f o u n d that C O a n d N O c h e m i s o r p t i o n o n t r a n s i t i o n metals have m a n y similar characteristics. H o w does the c h e m i s o r p t i o n of N 2 o n t r a n s i t i o n metals compare? I n recent years m u c h work has b e e n d o n e o n the c h e m i s o r p t i o n of N 2 o n N i surfaces b y e x p e r i m e n t a l a n d theoretical methods [4], i n c l u d i n g U P S [5-7], IES [5], L E E D , I R A S , TDS' [8,9] a n d ab initio cluster studies of N i N 2 [10,11]. T h e results of that surface science research suggest that the molecular N 2 is c h e m i s o r b e d in o n - t o p position a n d n o r m a l to the surface at low t e m p e r a t u r e a n d pressure. T h e major N 2 d e s o r p t i o n peak occurs at 186 K. The desorption heat is f o u n d to be a b o u t 35 k J / m o l . T h e U P S o n the N2-covered N i ( l l 0 ) surface looks very similar to the chemisorbed CO spectrum. I n the spectrum, two strong peaks 12.0 and 8.0 eV below the F e r m i level are noted. C o m p a r i s o n b e t w e e n the d a t a of A R U P S a n d the theoretical calculation of the N i N 2 complex shows that the b o n d i n g is p r i m a r i l y through a N z - 4 o - m e t a l b o n d with a small a m o u n t of Ni-d-N2-2~r b a c k - b o n d i n g [5]. These studies have provided a wealth of i n f o r m a t i o n a b o u t the n a t u r e of chemisorption a n d the a d s o r b a t e - s u b s t r a t e interaction. It was p o i n t e d out b y H o r n et al. [5] that the larger clusters should 0 0 3 9 - 6 0 2 8 / 8 7 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics P u b l i s h i n g Division)
Wu Yue, Cao Pei-lin / Molecular cluster calculations
L27
be investigated - either saturated configurations like Ni(N2) 4 or multi-metal clusters like N i s N 2 - so that the screening would become more realistic and a single N2-metal bond may better resemble the surface situation, and local density calculations needed to be done without the use of muffin-tins. The aim of the present work, in which the demands of both larger clusters and non-use of muffin-tins are satisfied, is to develop a deeper insight into the bonding mechanism of N 2 chemisorption on Ni surfaces. According to the results of the geometric analysis, the clusters chosen for the calculations are shown in fig. 1. The Ni9N 2 cluster, simulating the chemisorption of N 2 o n Ni(ll0), consists of five Ni atoms in the first layer, the four other Ni atoms in the second layer and one adsorbed N 2 molecule at the on-top site. Similarly the N i s N 2 and NiTN 2 simulate the N 2 adsorption on Ni(100) and (111) surfaces respectively. The N - N bond length, which is normal to the Ni surfaces, is equal to the free molecular value 1.09 A. In fig. 1, N a is the nitrogen atom near the Ni surfaces and N b is the other one. C2v, C4v
00 ®
0 0 O0 0@0 O0 oC o OOc
0
0
c-~Z)
Na
Fig. 1. Three cluster models for the calculation. (a) N i s N 2 on Ni (100), (b) NiTN 2 on N i ( l l l ) , (c) Ni9N 2 on Ni(ll0).
L28
Wu Yue, Cao Pei-lin / Molecular cluster calculations
and C6v symmetry are used for Ni9N2, N i s N 2 and NiTN 2 respectively. In the calculations, the vertical spacing h between the N a and the first Ni atom layer is varied to determine the optimized N i - N a bond distance and the structure sensitivity of the spectra. The theoretical method used in the calculations is Xc~-DVM, which has been described elsewhere [12-14]. From our results of the binding energy curves for these three clusters, the binding energies E d and the optimized N i - N a bond lengths dyi_N~ are obtained: NisN2: NiTN2: Ni9N2:
0.63 eV/molecule 0.67 eV/molecule 1.41 eV/molecule
1.85 A, 1.85 A, 1.80 A.
These binding energy results are somewhat larger than the experimental value mentioned above but the bond lengths are reasonable. In other theoretical calculations, dNi_Na, which is assumed to be the sum of covalent atomic radii, is larger than found by us. There are similar situations in our calculations for diatomic molecular adsorption on metal surfaces [1-3]. The theoretical ground-state valence levels for the three clusters are shown in figs. 2 and 3. It is clear that both 56(3al) and 46(2al) are bonding. The 56 is the only bonding level when the bond distance is large, but the 46 becomes stronger bonding when N 2 approaches the optimized site. The 46 levels shift 2.0 eV downward with respect to the level of the free N 2 molecule for all three Ni surfaces and the 56 levels shift down about 1.1 eV. The 46 is the main bonding level of N 2 chemisorption on Ni surfaces, but for CO adsorption the main bonding level is 56 [1-3]. This difference is caused by the different symmetry between N 2 and CO molecules [15]. The CO 40 and 5o are lone-pair-type orbitals on the oxygen and carbon atoms respectively. But the N 2 o orbitals both have approximately the same spatial extent along the molecular axis and are shared by both nitrogen atoms [15]. When the N 2 molecule is terminally bonded to a nickel atom, the metal electrons will equally overlap with both states. This will break the symmetry of the N 2 molecule, allowing two N 2 states to mix and form two new states which look like lone-pair states [6,16]. The l~7(lb~ + lb2) levels shift down only about 0.3 eV; they are nonbonding levels. There was an assumption in ref. [4] which indicated that the 1~7 level is unperturbed by bonding to the substrate. Our results show that this assumption is reasonable. It seems that this is due to the large distance between the adsorbed N 2 and Ni atoms around it. The variations of the gross atomic charges of N atoms, QN~ and QNb for NigNz, with the vertical spacing h are given in table 1. The QNb is rather stable but the Q~o changes largely with h. The shorter h, the smaller QN" (2p) and the larger QN- (2p). The occupation of the molecular orbitals of adsorbed
L29
Wu Yue, Cao Pei-lin / Molecular cluster calculations
EF -Z
-4 -6 -8
~ q e - -
"--
Ib~
le,
2bt /
-I0
/
/ "2(~-~-
-i,~
2(~. . . .
40-
/
f(Lt
-I~
-24
--l~t
--/al
--/~t
--36-
-z6 M'sNe
N~ zNz
NigNz
Ne r n o ~ _ z ~
Fig. 2. The ground-state valence levels for three clusters with their optimized h.
-2
Zbz-
-6
4~ J
30~I -........._
•- - - - - - . = = = = = ~ l b z
-8
~
Ifl
~4~
-/0
-/Z
2at/ -14
-
-z6
IC/.t
~
Zo
--
~s
. . . . . . . . .
41o
4'.s
30-
•CO)Z) '~
Fig. 3. The ground-state valence levels and their variation with spacing (h) for the Ni9N 2 cluster.
L30
Wu Yue, Cao Pei-lin / Molecular cluster calculations
Table 1 Gross atomic charges (QN a and QN b for N a and N b atoms respectively) and their variation with height h (in bohr) h (a.u.)
QN b (2s) QN" (2p) QN ~ (2s) QN" (2p)
3.0
3.4 ~)
3.5
4.0
4.5
12.0
1.67 3.44 1.54 3.71
1.68 3.42 1.57 3.64
1.68 3.41 1.58 3.63
1.68 3.40 1.61 3.55
1.68 3.39 1.64 3.50
1.69 3.39 1.68 3.40
a~ Optimized h value.
Table 2 Occupation numbers of the N z molecular orbitals
Free N 2 -Adsorbed N 2
3o
4o
50
lv
2v
2.0 2.0
2.0 2.0
2.0 1.83
4.0 4.0
0.0 0.48
N 2 at optimized position is given in table 2. These results show the donation-backdonation between the N 2 molecule and the substrate. The 5o donation is 0.17 electrons and the 2~r backdonation is 0.48 electrons. The backdonation is the main transfer of charge for N 2 chemisorption on Ni surfaces. The gross atomic charges of the Ni atoms of N i g N 2 and Ni 9 (which simulates the clean N i ( l l 0 ) surface) are given in table 3. We found that the atomic orbital occupation of Ni I has changed much more than for the other Ni atoms. It implies that the interaction between the adsorbed N 2 and the substrate is rather local [11]. The occupation number of Ni d orbitals is stable, which is similar to the other chemisorption systems on transition metal surfaces [1-3,14]. It seems that the backdonation electrons are mainly from occupied Ni 4s orbitals. The total DOS curves of Ni9N2, Ni 9 and their difference are shown in fig. 4, At - 11.9 and - 8.1 eV below E v, two strong additional peaks induced by the adsorbed N 2 molecule appear. They are 46 and 1~7 + 56 peaks respectively, which is in good agreement with the UPS results of N z / N i ( 1 1 0 ) Table 3 Gross atomic charge of N i atoms of Ni 9 and N i g N 2 clusters
3d 4s 3d NigN2 4s
Ni9
Ni I
Ni u
Ni m
Nilv
8.74 1.31 8.77 0.97
8.72 1.30 8.74 1.24
8.70 1.37 8.70 1.33
8.75 1.19 8.76 1.22
Wu Yue, Cao Pei-lin / Molecular cluster calculations
L31
~ A
i Fig. 4. The total density of states of Ni 9 (A), Ni9N 2 (B) and their difference (C). mentioned above. It is worth noting that there are several small peaks around 2.5 eV below E F in the difference curve. Analyzing the components of these peaks, it is found that they are derived from the___~xing between the N 2 2~r and metal orbitals. These peaks, named as (21r + d), are below E v. So N 2 2~r antibonding orbitals get some electrons from the occupied Ni orbitals, inducing the dissociation tendency of adsorbed N 2. For the N 2 - N i surface systems, the chemisorption of N 2 (on-top site) on Ni (110) is stronger than on (100) and (111) surfaces; the optimized N i - N bond length is 1.80 ,~. 46 is the main bond level but 177 has nonbonding character. The additional peaks in the total DOS induced by N 2 chemisorption are 46 and 56 + 1~7, in good agreement with UPS results. The interaction between N 2 and substrate is rather local. The backdonation is larger than the donation, and the increase of occupation of N 2 2¢7 orbitals induces the dissociation tendency of the adsorbed N 2 molecule. This work was supported by the Chinese N S F (Grant no. M407).
References [1] P.-L. Cao, Y. Wu, Y.-Q Chen and D.-J. Zheng, Appl. Surface Sci. 22/23 (1985) 452. [2J Chen Yun-qi, Zheng De-juan, Cao Pei-lin and Wu Yue, Acta Phys. Sinica 34 (1985) 1299.
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Wu Yue, Cao Pei-lin / Molecular cluster calculations
[3] Wu Yue, Cao Pei-lin, Chen Yun-qi and Zheng De-juan, Acta Phys. Sinica 34 (1985) 1306. [4] R.P. Messmer, Surface Sci. 158 (1985) 45. [5] K. Horn, J. Dinardo, W. Eberhardt, H.-J. Freund and E.W. Plummer, Surface Sci. 118 (1982) 465. [6] P.S. Bagus, C.R. Brundle, K. Hermann and D. Menzel, J. Electron Spectrosc. Related Phenomena 20 (1980) 253. [7] P.A. Dowben, Y. Sakisaka and T.N. Rhodin, Surface Sci. 147 (1984) 89. [8] M. Grunze, R K . Driscall, G.N. Burland, J.C.L. Cornish and J. Pritchard, Surface Sci. 89 (1979) 381. [9] H.A.C.M. Hendrickx, Surface Sci. 135 (1983) 81. [10] C.M. Kao and R.P. Messmer, Phys. Rev. B31 (1985) 4835. [11] H.J. Freund, R.P. Messmer, C.M. Kao and E.W. Plummer, Phys. Rev. B31 (1985) 4848. [12] D.E. Ellis and G.S. Painter, Phys. Rev. B2 (1970) 2887. [13] Cao Pei-lin, D.E. Ellis and A.J. Freeman, Phys. Rev. B25 (1982) 2124. [14] Cao Pei-lin, D.E. Ellis, A.J. Freeman, Qing-qi Zheng and S.D. Bader, Phys. Rev. B30 (1984) 4146. [15] C.M. Kao and R.P. Messmer, Chem. Phys. Letters 106 (1984) 183. [16] P.S. Bagus, K Hermann and M. SeeL J. Vacuum Sci. Technol. 18 (1981) 435.
Surface Science 179 (1987) L26-L32 North-Holland, Amsterdam
SURFACE SCIENCE LETTERS MOLECULAR CLUSTER CALCULATIONS FOR THE ANALYSIS OF N 2 CHEMISORIYrION ON NICKEL SURFACES W U Yue a n d C A O Pei-lin Department of Physics, Zhejiang University, Hangzhou, People's' Rep. of China Received 6 June 1986; accepted for publication 8 September 1986
Self-consistent Hartree-Fock-Slater molecular cluster calculations for the chemisorption of nitrogen on nickel surfaces have been performed. The clusters are NisN 2, NivN2 and NigN 2 for N 2 on Ni(100), (111) and (110) surfaces respectively, with the N-N distance equal to the free molecular value. The theoretical results for the binding energy show that the atop chemisorption on Ni(ll0) is stronger than on the other two surfaces. The optimized Ni-N bond length is 1.80 on Ni(ll0) and 1.85 A on the others. In the ground-state valence levels, 46 is the main bonding orbital. In the total DOS and difference curve for NigN 2, the 48 and 1~ + 56 peaks are located at 11.9 and 8.1 eV below E v, in good agreement with the UPS results. The N 2 2ff induced small peaks are around 2.5 eV below EF, which is related to the tendency of N 2 dissociation. The charge transfer in N 2 chemisorption is done by a donation-backdonation scheme.
I n our previous works we have analyzed the c h e m i s o r p t i o n of C O a n d N O o n t r a n s i t i o n metal surfaces b y X a - D V M (discrete variational m e t h o d ) [1-3]. T h e c h e m i s o r p t i o n of C O o n t r a n s i t i o n metal surfaces has been e m p l o y e d as the p r o t o t y p e system of molecular chemisorption. We f o u n d that C O a n d N O c h e m i s o r p t i o n o n t r a n s i t i o n metals have m a n y similar characteristics. H o w does the c h e m i s o r p t i o n of N 2 o n t r a n s i t i o n metals compare? I n recent years m u c h work has b e e n d o n e o n the c h e m i s o r p t i o n of N 2 o n N i surfaces b y e x p e r i m e n t a l a n d theoretical methods [4], i n c l u d i n g U P S [5-7], IES [5], L E E D , I R A S , TDS' [8,9] a n d ab initio cluster studies of N i N 2 [10,11]. T h e results of that surface science research suggest that the molecular N 2 is c h e m i s o r b e d in o n - t o p position a n d n o r m a l to the surface at low t e m p e r a t u r e a n d pressure. T h e major N 2 d e s o r p t i o n peak occurs at 186 K. The desorption heat is f o u n d to be a b o u t 35 k J / m o l . T h e U P S o n the N2-covered N i ( l l 0 ) surface looks very similar to the chemisorbed CO spectrum. I n the spectrum, two strong peaks 12.0 and 8.0 eV below the F e r m i level are noted. C o m p a r i s o n b e t w e e n the d a t a of A R U P S a n d the theoretical calculation of the N i N 2 complex shows that the b o n d i n g is p r i m a r i l y through a N z - 4 o - m e t a l b o n d with a small a m o u n t of Ni-d-N2-2~r b a c k - b o n d i n g [5]. These studies have provided a wealth of i n f o r m a t i o n a b o u t the n a t u r e of chemisorption a n d the a d s o r b a t e - s u b s t r a t e interaction. It was p o i n t e d out b y H o r n et al. [5] that the larger clusters should 0 0 3 9 - 6 0 2 8 / 8 7 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics P u b l i s h i n g Division)
Wu Yue, Cao Pei-lin / Molecular cluster calculations
L27
be investigated - either saturated configurations like Ni(N2) 4 or multi-metal clusters like N i s N 2 - so that the screening would become more realistic and a single N2-metal bond may better resemble the surface situation, and local density calculations needed to be done without the use of muffin-tins. The aim of the present work, in which the demands of both larger clusters and non-use of muffin-tins are satisfied, is to develop a deeper insight into the bonding mechanism of N 2 chemisorption on Ni surfaces. According to the results of the geometric analysis, the clusters chosen for the calculations are shown in fig. 1. The Ni9N 2 cluster, simulating the chemisorption of N 2 o n Ni(ll0), consists of five Ni atoms in the first layer, the four other Ni atoms in the second layer and one adsorbed N 2 molecule at the on-top site. Similarly the N i s N 2 and NiTN 2 simulate the N 2 adsorption on Ni(100) and (111) surfaces respectively. The N - N bond length, which is normal to the Ni surfaces, is equal to the free molecular value 1.09 A. In fig. 1, N a is the nitrogen atom near the Ni surfaces and N b is the other one. C2v, C4v
00 ®
0 0 O0 0@0 O0 oC o OOc
0
0
c-~Z)
Na
Fig. 1. Three cluster models for the calculation. (a) N i s N 2 on Ni (100), (b) NiTN 2 on N i ( l l l ) , (c) Ni9N 2 on Ni(ll0).
L28
Wu Yue, Cao Pei-lin / Molecular cluster calculations
and C6v symmetry are used for Ni9N2, N i s N 2 and NiTN 2 respectively. In the calculations, the vertical spacing h between the N a and the first Ni atom layer is varied to determine the optimized N i - N a bond distance and the structure sensitivity of the spectra. The theoretical method used in the calculations is Xc~-DVM, which has been described elsewhere [12-14]. From our results of the binding energy curves for these three clusters, the binding energies E d and the optimized N i - N a bond lengths dyi_N~ are obtained: NisN2: NiTN2: Ni9N2:
0.63 eV/molecule 0.67 eV/molecule 1.41 eV/molecule
1.85 A, 1.85 A, 1.80 A.
These binding energy results are somewhat larger than the experimental value mentioned above but the bond lengths are reasonable. In other theoretical calculations, dNi_Na, which is assumed to be the sum of covalent atomic radii, is larger than found by us. There are similar situations in our calculations for diatomic molecular adsorption on metal surfaces [1-3]. The theoretical ground-state valence levels for the three clusters are shown in figs. 2 and 3. It is clear that both 56(3al) and 46(2al) are bonding. The 56 is the only bonding level when the bond distance is large, but the 46 becomes stronger bonding when N 2 approaches the optimized site. The 46 levels shift 2.0 eV downward with respect to the level of the free N 2 molecule for all three Ni surfaces and the 56 levels shift down about 1.1 eV. The 46 is the main bonding level of N 2 chemisorption on Ni surfaces, but for CO adsorption the main bonding level is 56 [1-3]. This difference is caused by the different symmetry between N 2 and CO molecules [15]. The CO 40 and 5o are lone-pair-type orbitals on the oxygen and carbon atoms respectively. But the N 2 o orbitals both have approximately the same spatial extent along the molecular axis and are shared by both nitrogen atoms [15]. When the N 2 molecule is terminally bonded to a nickel atom, the metal electrons will equally overlap with both states. This will break the symmetry of the N 2 molecule, allowing two N 2 states to mix and form two new states which look like lone-pair states [6,16]. The l~7(lb~ + lb2) levels shift down only about 0.3 eV; they are nonbonding levels. There was an assumption in ref. [4] which indicated that the 1~7 level is unperturbed by bonding to the substrate. Our results show that this assumption is reasonable. It seems that this is due to the large distance between the adsorbed N 2 and Ni atoms around it. The variations of the gross atomic charges of N atoms, QN~ and QNb for NigNz, with the vertical spacing h are given in table 1. The QNb is rather stable but the Q~o changes largely with h. The shorter h, the smaller QN" (2p) and the larger QN- (2p). The occupation of the molecular orbitals of adsorbed
L29
Wu Yue, Cao Pei-lin / Molecular cluster calculations
EF -Z
-4 -6 -8
~ q e - -
"--
Ib~
le,
2bt /
-I0
/
/ "2(~-~-
-i,~
2(~. . . .
40-
/
f(Lt
-I~
-24
--l~t
--/al
--/~t
--36-
-z6 M'sNe
N~ zNz
NigNz
Ne r n o ~ _ z ~
Fig. 2. The ground-state valence levels for three clusters with their optimized h.
-2
Zbz-
-6
4~ J
30~I -........._
•- - - - - - . = = = = = ~ l b z
-8
~
Ifl
~4~
-/0
-/Z
2at/ -14
-
-z6
IC/.t
~
Zo
--
~s
. . . . . . . . .
41o
4'.s
30-
•CO)Z) '~
Fig. 3. The ground-state valence levels and their variation with spacing (h) for the Ni9N 2 cluster.
L30
Wu Yue, Cao Pei-lin / Molecular cluster calculations
Table 1 Gross atomic charges (QN a and QN b for N a and N b atoms respectively) and their variation with height h (in bohr) h (a.u.)
QN b (2s) QN" (2p) QN ~ (2s) QN" (2p)
3.0
3.4 ~)
3.5
4.0
4.5
12.0
1.67 3.44 1.54 3.71
1.68 3.42 1.57 3.64
1.68 3.41 1.58 3.63
1.68 3.40 1.61 3.55
1.68 3.39 1.64 3.50
1.69 3.39 1.68 3.40
a~ Optimized h value.
Table 2 Occupation numbers of the N z molecular orbitals
Free N 2 -Adsorbed N 2
3o
4o
50
lv
2v
2.0 2.0
2.0 2.0
2.0 1.83
4.0 4.0
0.0 0.48
N 2 at optimized position is given in table 2. These results show the donation-backdonation between the N 2 molecule and the substrate. The 5o donation is 0.17 electrons and the 2~r backdonation is 0.48 electrons. The backdonation is the main transfer of charge for N 2 chemisorption on Ni surfaces. The gross atomic charges of the Ni atoms of N i g N 2 and Ni 9 (which simulates the clean N i ( l l 0 ) surface) are given in table 3. We found that the atomic orbital occupation of Ni I has changed much more than for the other Ni atoms. It implies that the interaction between the adsorbed N 2 and the substrate is rather local [11]. The occupation number of Ni d orbitals is stable, which is similar to the other chemisorption systems on transition metal surfaces [1-3,14]. It seems that the backdonation electrons are mainly from occupied Ni 4s orbitals. The total DOS curves of Ni9N2, Ni 9 and their difference are shown in fig. 4, At - 11.9 and - 8.1 eV below E v, two strong additional peaks induced by the adsorbed N 2 molecule appear. They are 46 and 1~7 + 56 peaks respectively, which is in good agreement with the UPS results of N z / N i ( 1 1 0 ) Table 3 Gross atomic charge of N i atoms of Ni 9 and N i g N 2 clusters
3d 4s 3d NigN2 4s
Ni9
Ni I
Ni u
Ni m
Nilv
8.74 1.31 8.77 0.97
8.72 1.30 8.74 1.24
8.70 1.37 8.70 1.33
8.75 1.19 8.76 1.22
Wu Yue, Cao Pei-lin / Molecular cluster calculations
L31
~ A
i Fig. 4. The total density of states of Ni 9 (A), Ni9N 2 (B) and their difference (C). mentioned above. It is worth noting that there are several small peaks around 2.5 eV below E F in the difference curve. Analyzing the components of these peaks, it is found that they are derived from the___~xing between the N 2 2~r and metal orbitals. These peaks, named as (21r + d), are below E v. So N 2 2~r antibonding orbitals get some electrons from the occupied Ni orbitals, inducing the dissociation tendency of adsorbed N 2. For the N 2 - N i surface systems, the chemisorption of N 2 (on-top site) on Ni (110) is stronger than on (100) and (111) surfaces; the optimized N i - N bond length is 1.80 ,~. 46 is the main bond level but 177 has nonbonding character. The additional peaks in the total DOS induced by N 2 chemisorption are 46 and 56 + 1~7, in good agreement with UPS results. The interaction between N 2 and substrate is rather local. The backdonation is larger than the donation, and the increase of occupation of N 2 2¢7 orbitals induces the dissociation tendency of the adsorbed N 2 molecule. This work was supported by the Chinese N S F (Grant no. M407).
References [1] P.-L. Cao, Y. Wu, Y.-Q Chen and D.-J. Zheng, Appl. Surface Sci. 22/23 (1985) 452. [2J Chen Yun-qi, Zheng De-juan, Cao Pei-lin and Wu Yue, Acta Phys. Sinica 34 (1985) 1299.
L32
Wu Yue, Cao Pei-lin / Molecular cluster calculations
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